A nonlinear optical effect appearing remarkably with a strong light such as laser light irradiated to a material can be applied for wavelength conversion, strength modulation, switching, etc. There have been many prior disclosures about materials having the nonlinear optical effect in recent years.
Also, there have been many prior disclosures about optical recording media in response to demands for large capacity and high density of information recording media.
The recording density of the optical recording media heretofore used depends on the wavelength of light source, that is, a recording density increases in inverse proportion to square of the light source wavelength. therefore, there is a great demand for wavelength conversion device to obtain light sources of shorter wavelength.
It was pointed out that organic compounds would have extraordinary high performance in the wavelength conversion, particularly in second harmonic generation (hereinafter referred to as the SHG) on the basis of a second nonlinear optical effect, as compared with conventionally known inorganic materials such as lithium niobate (LiNbO.sub.3) and potassium dihydrogen phosphate (KDP). They are described, for example, in "Organic Nonlinear Optical Materials", edited by Masao Kato and Hachiro Nakanishi, C.M.C. Co., 1985.
Optical Nonlinearity of the organic compounds is originated in x electrons in molecules thereof. Second-order nonlinear molecular polarizability B can be made particularly great when the compounds have both electron donor substituent group and electron acceptor substituent group.
However, even if the nonlinear polarization is high at a molecular level, as typically shown by p-nitroaniline, it is often seen that no SHG is caused in state of crystal at all, or it is too little even if made. These are due to the fact that the molecular alignment of highly polarized molecules in crystal tends to be inverse symmetrical.
Known compounds having the SHG effect include, for example, 2-methyl-4-nitroaniline (MNA), 2-acetamide-4-nitro-N, N-dimethylaniline (DAN), 2-acetamide-4-nitro-1-pyrrolidinobenzene (PAN), 2-(.alpha.-methylbenzil)amino-5-nitropyridine (MBA-NP).
In order to make up the wavelength conversion device having high conversion efficiency with use of the materials mentioned above, a nonlinear polarization wave induced by a fundamental wave has to be phase-matched with a second harmonic wave caused by it. There have been proposed the wavelength conversion devices of optical waveguide type, such as a plane plate optical waveguide, a channel waveguide, and an optical fiber. These have been attracting attention as they could have high wavelength conversion efficiency. The reasons are that they can make the phase-matching in a relatively easy way with use of mode dispersion or Cerenkov radiation process, and as they also can have a high optical power density in the way that the optical wave is enclosed in a waveguide.
For example, a conventional optical fiber waveguide device has been made up in the way that a nonlinear optical material having high refractive index is melted and injected in hollow fiber formed of a material having lower refractive index. It then is crystallized to core hof single crystal in the Bridgman-Stockberger's method or similar way. For detail, as an example, see "Nonlinear Optical Properties of Organic Molecules and Crystals, Vol. 1, edited by D. S. Chemla and J. Zyss, Academic Press Inc., 1987.
There have been proposed fiber nonlinear optical devices having MNA, DAN, or m-DNB used for the core, these have the disadvantage that these organic nonlinear optical materials are not only hard to grow to single crystal, but also they tend to have defects, such as voids, cracks, and rotation of crystal axes, caused in the single crystal formed.
In the way that single crystal of nonlinear optical material grow in the hollow fiber, the above nonlinear optical materials have the molecular polarization axis made to become in parallel with the fiber axis. This is hard to form the wavelength conversion device having the high conversion efficiency obtained with full use of the nonlinear optical effect inherent to the materials.